1. Field of the Invention
This invention relates generally to a switching apparatus for transmitting a plurality of signals, and more particularly to an improved construction for a pushbutton call transmitter.
2. Description of the Prior Art
The use of pushbutton calling devices associated with telephone subscribers instruments equipped for operation in automatic telephone systems is well known. Some of the earliest versions of this signalling technique where utilized even prior to the development of the conventional dial, now long used for controlling switching paths in the telephone network.
Most recently it has become conventional to equip telephone subscribers instruments for touchcalling service with pushbutton call transmitters using oscillator circuitry capable of producing on a selective basis two tone signals simultaneously in response to a single operation of one of a group of included pushbuttons. Each tone pair generated is recognizable by the telephone central office as representative of a single selected digit. Two particular disadvantages of this form of signalling was a requirement for inclusion of a common switch and the need for more than one frequency selection switch contact associated with each pushbutton. Frequency selection was accomplished by including two contacts per pushbutton or the utilization of mechanical coding linkages. The common switching function was accomplished by mechanically coupling all pushbuttons to a single switch, or by adding switch contacts to each pushbutton. Such methods of code selection and common switch operation were costly to implement from a mechanical viewpoint. Likewise, the reliability of such arrangements has also been a problem.
Advances in integrated circuit technology have produced digital touch-calling encoders which generate telephone standard multi-frequency tones in response to digital inputs. Such encoders are taught by U.S. Pat. No. 3,761,642 to Paul V. Lind, and exemplified by the MK5085N integrated tone dialer, available commercially by the Mostek Corporation. These devices require only a single make contact to produce a multi-frequency tone required for dialing. This advance in dialing technology has lead to the incorporation of simpler more reliable pushbutton switching arrangements to telephone instruments which are commonly known as Class A keyboards and more typically used with hand-held calculators and computer keyboard devices.
Realistically, telephone call transmitters must be manufactured to withstand thousands of individual pushbutton switching operations over the life of the telephone instrument. A failure in any one of the included pushbuttons would render the entire call transmitter useless. It becomes desirable not only to manufacture a simpler pushbutton call transmitter for reliability, but also to be able to assemble the call transmitter with a certain degree of modularity, where, if one part of the assembly, i.e. the switching contacts or the electronic components would fail only that element of the assembly would be discarded and replaced and not the entire assembly. Further this modularity may be also used to advantage in manufacturing a standard sealed contact switching unit, to which many different types of pushbuttons, and faceplates may be applied, such as special vandal proof pushbuttons and faceplates for coin operated telephones. Additionally, if the electronic components are carried on a standard circuit board adapted to electrically connect to the contact switching unit it would be a matter of substituting one type of circuit board of a particular telephone operating characteristic for another. An example of this technique would be in converting from DTMF dialing to electronic pulse dialing and vice versa.
Accordingly, it is the object of the present invention to provide a pushbutton call transmitter for use in telephone instruments which requires only a single make switch contact associated with each pushbutton.
It is a further object of the present invention to provide an improved construction for a pushbutton call transmitter which exhibits a degree of modularity between the functional elements of the assembly.